Chemical Composition for Tri-Layer Removal

ABSTRACT

A method includes forming a tri-layer. The tri-layer includes a bottom layer; a middle layer over the bottom layer; and a top layer over the middle layer. The top layer includes a photo resist. The method further includes removing the top layer; and removing the middle layer using a chemical solution. The chemical solution is free from potassium hydroxide (KOH), and includes at least one of a quaternary ammonium hydroxide and a quaternary ammonium fluoride.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. patent application: Application Ser. No. 62/551,985, filed Aug. 30,2017, and entitled “Chemical Composition for Tri-Layer Removal;” whichapplication is hereby incorporated herein by reference.

BACKGROUND

In the formation of integrated circuits, the components of theintegrated circuit devices need to be patterned to form desirableshapes. A typical patterning process includes a photo lithographyprocess, which includes coating a photo resist over a target layer thatis to be patterned, light-exposing the photo resist using a lithographymask, developing the photo resist, and using the developed photo resistas an etching mask to etch the target layer. As a result, the layout ofthe developed photo resist is transferred to the underlying layer. Thephoto resist is then removed.

In some situations, after the photo resist is patterned, it may be foundthat the patterned photo resist has defects. The patterned photo resistis thus removed, and a new photo resist is applied and patterned again.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediatestages in the formation of metal lines and vias in accordance with someembodiments.

FIG. 12 illustrates a diagram of siloxane in accordance with someembodiments.

FIG. 13 illustrates the dissociated tetramethylammonium hydroxide (TMAH)in accordance with some embodiments.

FIG. 14 illustrates the dissociated choline hydroxide in accordance withsome embodiments.

FIG. 15 illustrates an exemplary siloxane hydrolysis of TMAH inaccordance with some embodiments.

FIG. 16 illustrates the etching rate of a middle layer as a function ofthe temperature of the chemical solution in accordance with someembodiments.

FIG. 17 illustrates the dissociated tetramethylammonium fluoride (TMAF)in accordance with some embodiments.

FIG. 18 illustrates the dissociated tetrabutylammonium tetrafluoroboratein accordance with some embodiments.

FIG. 19 illustrates an exemplary process in which TMAF reacts withsiloxane in accordance with some embodiments.

FIG. 20 illustrates a diagram of a portion of an exemplary middle layerin accordance with some embodiments.

FIG. 21 illustrates a process flow in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A method of performing rework on tri-layers used for photo lithographyprocess and the chemical solution used for the rework are provided inaccordance with various exemplary embodiments. The intermediate stagesof the rework are illustrated in accordance with some embodiments. Somevariations of some embodiments are discussed. Throughout the variousviews and illustrative embodiments of the present disclosure, likereference numbers are used to designate like elements.

FIGS. 1 through 11 illustrate the cross-sectional views of intermediatestages in the formation of metal lines and vias in accordance with someembodiments of the present disclosure. The steps shown in FIGS. 1through 11 are also reflected schematically in the process flow 200 asshown in FIG. 21. It is appreciated that the process shown in FIGS. 1through 11 is an exemplary embodiment for performing photo lithographyrework of tri-layers, and the embodiments may be applied on the photolithography processes for etching other features including, and notlimited to, semiconductor substrates, metal layers, etc. For example,the photo lithography process in accordance with the embodiment of thepresent disclosure may be used for the formation of semiconductor fins,on which Fin Field-Effect Transistors (FinFETs) are formed.

FIG. 1 illustrates a portion of wafer 100, which includes substrate 10and a plurality of layers formed over substrate 10. Substrate 10 may beformed of a semiconductor material such as silicon, silicon germanium,or the like. In accordance with some embodiments of the presentdisclosure, substrate 10 is a crystalline semiconductor substrate suchas a crystalline silicon substrate, a crystalline silicon carbonsubstrate, a crystalline silicon germanium substrate, a III-V compoundsemiconductor substrate, or the like. Active devices 12, which mayinclude transistors therein, are formed at a top surface of substrate10.

Dielectric layer 14 is formed over substrate 10. In accordance with someembodiments of the present disclosure, dielectric layer 14 is anInter-Metal Dielectric (IMD) or an Inter-Layer Dielectric (ILD), whichmay be formed of a dielectric material. The dielectric constant (kvalue) of dielectric layer 14 may be lower than 3.8, lower than about3.0, or lower than about 2.5, for example. In accordance with someembodiments of the present disclosure, conductive features 16, which maybe metallic features such as copper lines or tungsten plugs, are formedin dielectric layer 14. Etch stop layer 26 is formed over dielectriclayer 14. Etch stop layer 26 may be formed of a dielectric material suchas silicon carbide, silicon nitride, or the like.

Dielectric layer 28 is further formed over etch stop layer 26.Dielectric layer 28 may be an IMD layer, which is formed of a dielectricmaterial having a dielectric constant (k value) lower than 3.8, lowerthan about 3.0, or lower than about 2.5, for example. In accordance withalternative embodiments of the present disclosure, dielectric layer 28is a non-low-k dielectric layer having a k value higher than 3.8.

In accordance with alternative embodiments of the present disclosure,layer 28 is a semiconductor substrate, wherein the subsequent processsteps may be used to form Shallow Trench Isolation (STI) regions, forexample. In accordance with these embodiments, there may not beadditional layers underlying layer 28. Throughout the description, layer28 is also referred to as a target layer that is to be etched, in whicha plurality of patterns is to be formed in accordance with embodimentsof the present disclosure.

Over low-k dielectric layer 28 resides one or a plurality of hard masks.It is appreciated that depending on the process, different number ofhard masks may be adopted. In accordance with some exemplaryembodiments, the hard masks include layers 30, 32, and 34. Dielectrichard mask 30, which may be formed of silicon oxide (such astetraethylorthosilicate (TEOS) oxide), Nitrogen-Free Anti-ReflectiveCoating (NFARC, which is an oxide), silicon carbide, silicon oxynitride,or the like. The formation methods include Plasma Enhance Chemical VaporDeposition (PECVD), High-Density Plasma (HDP) deposition, or the like.

Metal hard mask 32 is formed over dielectric hard mask 30. In accordancewith some embodiments of the present disclosure, metal hard mask 32 isformed of titanium nitride, titanium, tantalum nitride, tantalum, or thelike. The formation methods may include Physical Vapor Deposition (PVD),Radio Frequency PVD (RFPVD), Atomic Layer Deposition (ALD), or the like.

In accordance with some embodiments of the present disclosure,dielectric hard mask layer 34 is formed over metal hard mask 32. Inaccordance with alternative embodiments, dielectric hard mask layer 34is not formed. Dielectric hard mask layer 34 may be formed of a materialselected from the same candidate material for forming dielectric hardmask layer 30, and may be formed using a method that is selected fromthe same group of candidate methods for forming dielectric hard masklayer 30. Dielectric hard masks 30 and 34 may be formed of the samematerial, or may be formed of different materials.

Mandrel layer 36 is formed over dielectric hard mask 34. In accordancewith some embodiments of the present disclosure, mandrel layer 36 isformed of amorphous silicon or another material that has a high etchingselectivity relative to the underlying dielectric hard mask 32.

Over mandrel layer 36, a tri-layer is formed, which includes bottomlayer 38 (sometimes referred to as a under layer), middle layer 40 overbottom layer 38, and top layer 42 over middle layer 40. The respectivestep is illustrated as step 202 in the process flow 200 shown in FIG.21. In accordance with some embodiments of the present disclosure,bottom layer 38 includes carbon, hydrogen, and oxygen, and is formed ofa material similar to photo resist. Furthermore, all of bottom layer 38is cross-linked, and hence is different from typical photo resists usedfor light exposure. Alternatively stated, bottom layer 38 is not able tohave some portions removed through photo exposure and development.Bottom layer 38 functions as a Bottom Anti-Reflective Coating (BARC)when top layer 42 is light-exposed.

Middle layer 40 may be formed of a material including silicon andoxygen, and hence has some of its property similar to silicon oxide. Forexample, middle layer 40 may include between about 30 weight percent andabout 60 weight percent silicon oxide. On the other hand, middle layer40 further includes organic groups bonded to silicon and oxygen as apolymer. In accordance with some embodiments of the present disclosure,middle layer 40 includes siloxane, which includes the chain of Si—O—Sibonds. Middle layer 40 also has a glass-like surface, and may densifyover time. For example, FIG. 12 illustrates a part of an exemplarycomposition of middle layer 40, in which a first silicon atom, an oxygenatom, and a second silicon atom are bonded to form a part of siloxane.It is appreciated that although two silicon atoms are illustrated as anexample, more silicon atoms may be bonded through oxygen atoms to form amuch larger structure, as also illustrated in FIG. 20. As shown in FIG.12, the oxygen atoms may be terminated by function groups such as ethylgroups, methyl groups, or the like. In accordance with some embodimentsof the present disclosure, middle layer 40 has a thickness in the rangebetween about 200 Å and about 350 Å.

Referring back to FIG. 1, top layer 42 is formed of a photo resist,which may include organic materials. Top layer 42 is applied as ablanket layer, which includes the dashed portions and solid portions asshown in FIG. 1. Next, a photo lithography process is performed, andphoto lithography mask 44 is used to perform a light exposure on toplayer 42. The respective step is illustrated as step 204 in the processflow 200 shown in FIG. 21. Photo lithography mask 44 includes opaqueportions and transparent portions, and hence some portions (such as thedashed portions) of top layer 42 are exposed, and other portions (suchas solid portions) are not exposed. After the light exposure, someportions (such as the exposed portions) are removed in a developmentstep, and other portions (such as unexposed portions) remain after thedevelopment step. The respective development step is also illustrated asstep 204 in the process flow 200 shown in FIG. 21. It is realized thatthe illustrated lithography mask 44, which is transmission-type, ismerely an example to show how patterns are formed, and other types oflithography masks such as reflective-type masks may also be used.

After the development, the patterned top layer 42 is inspected to checkthe patterns against specification. For example, the widths (criticaldimensions), the straightness, and the width uniformity may beinspected. Through the inspection, the patterned top layers 42 on somewafers may be determined as having defects, and rework is needed.Accordingly, the steps shown in FIGS. 2 through 4 are performed torework on the top layer. The patterned top layers 42 on some otherwafers may be determined as meeting the specification. Accordingly, therework steps as shown in FIGS. 2 through 4 are skipped, and the processcontinues starting from the step shown in FIG. 5 for these wafers.

The top layer 42 that needs to be reworked is first removed. Theresulting structure is shown in FIG. 2. The respective step isillustrated as step 206 in the process flow 200 shown in FIG. 21. Inaccordance with some embodiments of the present disclosure, the removalof top layer 42 includes a wet etch process using a solvent. Forexample, a mixture of 70% Propylene glycol monomethylether and 30%propylene glycol monomethylether acetate (in combination known as OK73,which is a photo resist thinner) may be used. In accordance withalternative embodiments of the present disclosure, the top layer 42 isremoved in an ashing process, wherein oxygen (O₂) is used to remove thetop layer 42.

Middle layer 40 is then removed in a wet etching process, and theetching and the respective chemical solution are represented usingarrows 41 as shown in FIG. 2. The resulting structure is shown in FIG.3. The respective step is illustrated as step 208 in the process flow200 shown in FIG. 21. The removal of middle layer 40 stops on bottomlayer 38, so that the top surface of bottom layer 38 is exposed. In theremoval of middle layer 40 (with or without top layer 42), wafer 100 issubmerged in the chemical solution, until middle layer 40 is fullyremoved.

The chemical solution for removing middle layer 40 includes either analkali, a fluoride, or a mixture of both. The alkali may be a quaternaryammonium hydroxide in accordance with some embodiments of the presentdisclosure. The fluoride may be a quaternary ammonium fluoride inaccordance with some embodiments of the present disclosure. The chemicalsolution further includes an organic solvent and water. Both of thequaternary ammonium hydroxide and the quaternary ammonium fluoride havethe function of breaking the bonds in middle layer 40 and hence etchingmiddle layer 40. The organic solvent has the function of stabilizing themolecules de-bonded from middle layer 40, and preventing the moleculesthat are de-bonded from re-bonding again to re-form the siloxane (whichmeans re-forming (depositing) middle layer 40).

In accordance with some embodiments of the present disclosure, thequaternary ammonium hydroxide used in the chemical solution is selectedfrom tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide,tetrapropylammonium hydroxide, tetrabutylammonium hydroxide,methyltripropylammonium hydroxide, methyltributylammonium hydroxide,benzyltrimethylammonium hydroxide, benzyltriethylammonium hydroxide,choline hydroxide ((2-Hydroxyethyl)trimethylammonium) hydroxide, andcombinations thereof.

The quaternary ammonium hydroxide is an alkali, and the hydroxide in thequaternary ammonium hydroxide provides an OH⁻ molecule when dissociated.For example, FIG. 13 illustrates the dissociated TMAH, wherein themolecules of TMA and OH⁻ that have been dissociated from each other, areillustrated. FIG. 14 illustrates the dissociated choline hydroxide,wherein the molecules of choline and OH⁻ that have been dissociated fromeach other, are illustrated. The OH⁻ reacts with siloxane throughsiloxane hydrolysis to form silanol, and during this process, the middlelayer 40, which is formed of siloxane, is broken apart and hence isetched.

FIG. 15 illustrates an exemplary siloxane hydrolysis of TMAH inaccordance with some embodiments of the present disclosure. Theillustrated exemplary siloxane (which is the component of middle layer40) includes Si—O—Si bonds. Although one Si—O—Si bond is illustrated asan example, there may be multiple Si—O—Si bonds to extend the chain. TheOH⁻ molecule attacks one of the Si—O bond, and forms a bond with the Siatom. As a result, the silanol, which includes a silicon atom bonded toan OH group, is formed.

The chemical solution for etching middle layer 40 preferably does notinclude the alkali with small molecule weight, for example, withmolecule weight lower than about 60. The alkali excluded from middlelayer 40 may include KOH, NaOH, NH₄OH, etc. The alkali with smallmolecule weight has a low steric hindrance, and hence is prone topenetrating into bottom layer 38 since bottom layer 38 is porous. Thealkali, if penetrates into bottom layer 38, may be released during thesubsequent photo lithography process (FIG. 4), and may react with thephoto resist acid in the photo resist, and hence may adversely affectthe subsequent photo lithography process. On the other hand, thequaternary ammonium hydroxide has high steric hindrance, and does notpenetrate into bottom layer 38.

In accordance with some embodiments of the present disclosure, thequaternary ammonium fluoride is selected from ammonium fluoride,ammonium bifluoride, tetramethylammonium fluoride (TMAF),tetrabutylammonium fluoride, tetraethylammonium hydroborontetrafluoride, tetrabutylammonium tetrafluoroborate, tetraethylammoniumtetrafluoroborate, and combinations thereof.

The quaternary ammonium fluoride provides a F⁻ ion, a BF₄ ⁻ molecule, orthe like when dissociated. For example, FIG. 17 illustrates thedissociated TMAF, and the TMA molecule and F⁻ ion dissociated from theTMAF are illustrated. FIG. 18 illustrates the dissociatedtetrabutylammonium tetrafluoroborate, and the molecules oftetrabutylammonium and tetrafluoroborate (BF₄ ⁻), which have beendissociated from each other. The F⁻ ion or the BF₄ ⁻ molecule reactswith siloxane, and during this process, the middle layer 40, whichincludes siloxane, is broken apart and hence is etched.

FIG. 19 illustrates an exemplary process in which a F⁻ ion (for example,from the dissociated TMAF) reacts with the siloxane in middle layer 40in accordance with some embodiments of the present disclosure. Theillustrated exemplary siloxane is attacked by a F⁻ ion. Further with theH₃O+ (in water) joining the reaction, the F⁻ ion is bonded with asilicon atom. Hence, siloxane is etched. It is appreciated that thefluorine atom bonded to silicon may further be replaced by the OH groupin water, and the F⁻ ion is separated as a discrete (un-bonded) ionagain. Accordingly, the separated F⁻ ion may join the further etching ofsiloxane. This means that the F⁻ ion may repeatedly join the reaction toetch more siloxane. Accordingly, a small amount of F⁻ ion is adequatefor the reaction, and the efficiency of the F⁻ ion in the etching ofmiddle layer 40 is high.

In accordance with some embodiments of the present disclosure, thechemical solution for etching middle layer 40 preferably does notinclude the fluoride having a small molecule weight such as HF. Forexample, the fluorides with molecule weights lower than about 60 are notused in the etching solution. Similarly, the fluorides with smallmolecule weight may have small steric hindrance, and hence are prone topenetrating into bottom layer 38 since bottom layer 38 is porous. Thefluoride penetrated into bottom layer 38 may be released during thesubsequent photo lithography process (FIG. 4) to damage the subsequentlyformed middle layer, and adversely affects the subsequent photolithography process. On the other hand, the quaternary ammoniumfluorides have high steric hindrance, and do not penetrate into bottomlayer 38.

In accordance with some embodiments of the present disclosure, eitherone of quaternary ammonium hydroxide and quaternary ammonium fluoride isused in the chemical solution for etching middle layer 40. In accordancewith some embodiments of the present disclosure, both quaternaryammonium hydroxide and quaternary ammonium fluoride are adopted in thechemical solution for etching middle layer 40. Since a significant(weight) percentage of middle layer 40 is silicon oxide, quaternaryammonium fluoride may be used as an efficient etchant. On the otherhand, quaternary ammonium fluoride is not very efficient in etchinglarge organic groups such as benzyl groups, while quaternary ammoniumhydroxide is efficient in the etching of large organic groups.Accordingly, quaternary ammonium hydroxide and quaternary ammoniumfluoride may compensate for each other's etching ability to make theetching of middle layer 40 more efficient. FIG. 20 illustrates a diagramof a portion of an exemplary middle layer 40. The left portion inrectangular region 40A is Chromorphore, which has a large organic group,and it is efficient to etch this portion using TMAH. The right portionin rectangular region 40B is a methylsiloxane polymer, and it isefficient to etch this portion using TMAF.

Table 1 illustrates the results of etching 12 samples using chemicalsolutions including TMAH, TMAF, and ethylene glycol (EG, as an organicsolvent). The chemical solutions for etching the 12 samples havedifferent combinations of weight percentages of TMAH, TMAF, and ethyleneglycol.

TABLE 1 Sample TMAH TMAF EG Etching rate # percentage percentagePercentage (Å/minute) 1 1.5% 0.25% N/A 350 2 0.5%  1.5% N/A 500 3 1.5% 1.5% N/A 400 4 1.5% 0.25%  1% 350 5 0.5%  1.5%  1% 500 6 1.5%  1.5%  1%400 7 1.5% 0.25%  5% 350 8 0.5%  1.5%  5% 500 9 1.5%  1.5%  5% 400 101.5% 0.25% 10% 350 11 0.5%  1.5% 10% 500 12 0.5%  1.5% 10% 400

In Table 1, the etching rates (Å/minute) of the samples are illustrated.The results indicate that the ratio of the weight percentage of TMAH tothe weight percentage of TMAF (shown as TMAH:TMAF hereinafter) affectsthe etching rate significantly. For example, it was found that when theratio TMAH:TMAF is about 1:3, the highest etching rate 500 Å/minute canbe reached, which results in the highest throughput in production.Accordingly, in accordance with some exemplary embodiments of thepresent disclosure, the ratio of TMAH:TMAF in the etching solution isclose to 1:3, for example, in the range between about 1:1 and about 1:5.On the other hand, the percentage of the organic solvent in the chemicalsolution does not significantly affect the etching rate. However, thetype and the weight percentage of the organic solvent may play a role inimproving the etching selectivity between middle layer 40 and bottomlayer 38.

Referring back to FIG. 15, as a by-product of the siloxane hydrolysis,in addition to the formed silanol, there is a molecule having an oxygenatom with a dangling bond. This molecule is unstable, and is prone toreaction with other similar molecules to form siloxane again, which maybe re-deposited on bottom layer 38 (FIG. 3) to re-form middle layer 40.To solve this problem, an organic solvent, which has the function ofstabilizing the molecules to prevent the molecules from re-bond witheach other and re-deposit, is adopted in the chemical solution foretching middle layer 40. In accordance with some embodiments of thepresent disclosure, the selected organic solvent(s) in the chemicalsolution have the boiling temperature higher than about 100° C. since inthe etching of middle layer 40, the temperature may be elevated, forexample, to a temperature in the range between about 35° C. and about60° C. In accordance with some embodiments of the present disclosure,since the chemical solution includes mainly water, the selected organicsolvents are also water soluble.

In accordance with some embodiments of the present disclosure, thecandidate organic solvents include Tetrahydrofurfuryl Alcohol (THFA),Butyl Diglycol (BDG), ethylene glycol (EG), propanol, glycerin,sulfolane, Dimethyl Sulfoxide (DMSO), Triethanolamine (TEA), orcombinations thereof. Also, the organic solvents may exclude someorganic solvents having small molecule weights such as ethers andprimary amines. Accordingly, the organic solvents that includeTetrahydrofuran (THF), propylene glycol monomethyl ether (PGME), and/ormonoethanolamine (MEA) may be excluded. Experiment results also revealedthat these organic solvents caused middle layer 40 to be lifted offrather than etched, hence may cause the damage of the underlying bottomlayer 38 (FIG. 3).

Experimental results revealed that the etching rate is related to thetemperature of the chemical solution, and the etching rate increaseswhen the temperature increases. For example, FIG. 16 illustrates theetching rate of the middle layer as a function of the temperature of thechemical solution. The chemical solution includes 0.5 weight percentTMAH, 1.5 weight percent TMAF, and 2 weight percent EG. The experimentalresults indicate that when temperature reaches close to about 40° C.,the etching rate may achieve the desirable value of about 400 Å/minuteor higher.

Referring back to FIGS. 2 and 3, when the middle layer 40 is etchedusing the chemical solution, the bottom layer 38 is not etched, andpreferably has as small damage as possible, so that bottom layer 38 maybe reused in the subsequent photo lithography process as shown in FIG.4. The etching selectivity in accordance with some embodiments of thepresent disclosure is high, for example, higher than about 100, and theetching selectivity is the ratio of the etching rate of middle layer 40to the etching rate of bottom layer 38. The high etching selectivity maybe achieved by selecting an appropriate type of solvent, and keeping thepercentage of the solvent low. In accordance with some embodiments ofthe present disclosure, the solvent has a weight percentage smaller thanabout 20 percent, and may be in the range between about 10 percent andabout 15 percent. The remaining components in the chemical solution aremainly water, which may have a weight percentage higher than about 70percent. The weight percentage of water may be close to about 80percent, or in the range between about 75 percent and about 85 percent.

FIGS. 4 and 5 illustrate the rework of the photo resist. In accordancewith some embodiments of the present disclosure, as shown FIG. 4, middlelayer 140 and top layer 142 are formed. The respective step isillustrated as step 210 in the process flow 200 shown in FIG. 21. Thematerial of middle layer 140 and top layer 142 may be essentially thesame as or similar to that of middle layer 40 and top layer 42,respectively, as shown in FIG. 1. Next, a light exposure is performed ontop layer 142, followed by a development step to remove the dashedportions of top layer 142. The respective step is illustrated as step212 in the process flow 200 shown in FIG. 21. Photo lithography mask 144is used for the light exposure of top layer 142. In accordance with someembodiments of the present disclosure, photo lithography mask 144 has anidentical pattern as, or may be the same one, as the photo lithographymask 44 shown in FIG. 1. The exposed top layer 142 is then developed,and the dashed portions are removed. The respective step is alsoillustrated as step 212 in the process flow 200 shown in FIG. 21. Aninspection is then performed to check the quality of the patterned toplayer 142. If the quality (such as line width, straightness, uniformity,etc.) of top layer 142 meets the specification, top layer 142 will beused to etch underlying layers. If the quality of top layer 142 does notmeet the specification, another rework will be performed by repeatingthe steps shown in FIGS. 2 through 4. The rework is repeated until thefinal top layer meets specification.

Next, top layer 142 is used as an etching mask to etch the underlyinglayer. FIG. 5 illustrates the cross-sectional view of an intermediatestage, in which middle layer 140 has been patterned. Next, bottom layer38 is patterned using patterned layers 140 and 142 as an etching mask,and the remaining patterned tri-layer including layers 38, 140, and 142are used as an etching mask to etch mandrel layer 36. The remainingportions of mandrel layer 36 are referred to as mandrels 136 (FIG. 6)hereinafter. The respective step is illustrated as step 214 in theprocess flow 200 shown in FIG. 21. The remaining portions of thetri-layer are then removed, and the resulting structure is shown in FIG.6.

Referring to FIG. 7, spacer layer 46 is formed in accordance with someembodiments of the present disclosure. The respective step isillustrated as step 216 in the process flow 200 shown in FIG. 21. Spacerlayer 46 is a conformal layer, with the thickness T1 of its horizontalportions and the thickness T2 of its vertical portions being close toeach other, for example, with a difference between thicknesses T1 and T2smaller than about 20 percent of thickness T1. An anisotropic etching isthen performed to remove the horizontal portions of spacer layer 46,while the vertical portions of spacer layer 46 remain, and are referredto as spacers 146 hereinafter. The resulting structure is shown in FIG.8. The respective step is illustrated as step 218 in the process flow200 shown in FIG. 21. The resulting spacers 146 thus have a pitch equalto a half of the pitch of mandrels 136, and hence the correspondingprocess is referred to as a double-patterning process. Mandrels 136 arethen removed, and the resulting structure is shown in FIG. 9. Openings50 are thus formed between mandrels spacers 146.

In accordance with some embodiments of the present disclosure, spacers146 are used as an etching mask to etch the underlying dielectric hardmask 34 and metal hard mask 32. The respective step is illustrated asstep 220 in the process flow 200 shown in FIG. 21.

Next, dielectric hard mask 34 and metal hard mask 32 are used as anetching mask to etch hard mask 30. Spacers 146 may be consumed in thisprocess. The resulting structure is shown in FIG. 10. In FIG. 10, thepatterned hard mask 32 is also used as an etching mask to etch theunderlying low-k dielectric layer 28 and etch stop layer 26, so thattrenches 52 are formed. Additional process steps are also performed todefine and etch low-k dielectric layer 28 to form via openings 54underlying trenches 52. Although trenches 52 and via openings 54 havethe same widths in the illustrated plane, in a vertical planeperpendicular to the illustrated plane, via openings 54 have smallerwidths than trenches 52.

FIG. 11 illustrates the filling of trenches 52 and via openings 54 (FIG.10) to form metal lines 56 and vias 58, respectively. The respectivestep is illustrated as step 222 in the process flow 200 shown in FIG.21. In accordance with some embodiments of the present disclosure, theformation process includes a dual damascene process, wherein aconductive barrier layer such as titanium nitride, titanium, tantalumnitride, tantalum, or the like is formed on the sidewalls and thebottoms of trenches 52 and via openings 54. The remaining portions oftrenches 52 and via openings 54 are then filled with a filling metalsuch as copper or a copper alloy. A Chemical Mechanical Polish (CMP) isthen performed to remove excess portions of the barrier layer and thefilling metal, forming metal lines 56 and vias 58 as shown in FIG. 11.Metal lines 56 and vias 58 are electrically connected to the underlyingconductive features 16. In subsequent steps, an etch stop layer (notshown) is formed over dielectric layer 28 and metal lines 56, followedby the formation of another low-k dielectric layer, and the steps shownin FIGS. 1 through 11 may be repeated to form more metal lines and vias.

In accordance with some embodiments of the present disclosure, asillustrated above, target layer 28 is a dielectric layer, and theprocess steps of the present disclosure are used to form metal lines inthe dielectric layer. In accordance with alternative embodiments of thepresent disclosure, target layer 28 is formed of a semiconductormaterial such as a semiconductor substrate. Accordingly, the processstep shown in FIGS. 1 through 11 may be used to form trenches in targetlayer 28, and the trenches can be filled with a dielectric material toform Shallow Trench Isolation (STI) regions. In accordance withalternative embodiments of the present disclosure, the process steps inaccordance with the embodiments of the present disclosure may be used toetch various features such as dielectric features, semiconductorfeatures, or metal features to form dielectric lines, semiconductorlines, or metal lines.

The embodiments of the present disclosure have some advantageousfeatures. By including quaternary ammonium hydroxide and/or quaternaryammonium fluoride in the chemical solution for etching the middle layerin a tri-layer, due to the steric hindrance of quaternary ammoniumhydroxide and quaternary ammonium fluoride, the quaternary ammoniumhydroxide and quaternary ammonium fluoride do not penetrate into theunderlying bottom layer. Furthermore, the chemicals with small moleculeweights are not used in the chemical solution, and hence in the reworkof the tri-layer, no chemical adversely penetrates into the bottomlayer. The adverse effect of these small-molecule weight chemicals tothe subsequent rework is thus avoided. In addition, quaternary ammoniumhydroxide and quaternary ammonium fluoride (especially the combinationof quaternary ammonium hydroxide and quaternary ammonium fluoride) havehigh etching rates for etching the middle layer. Also, the etchingselectivity (the ratio of the etching rate of the middle layer to theetching rate of the bottom layer) is high, hence the damage to thebottom layer is minimized, and the bottom layer can be reused. Thissignificantly reduces the manufacturing cost.

In accordance with some embodiments of the present disclosure, a methodincludes forming a tri-layer, the tri-layer comprising: a bottom layer;a first middle layer over the bottom layer; and a first top layer overthe first middle layer, wherein the first top layer comprises a photoresist; removing the first top layer; and removing the first middlelayer using a chemical solution, wherein the chemical solution is freefrom KOH, and comprises at least one of a quaternary ammonium hydroxideand a quaternary ammonium fluoride. In an embodiment, the bottom layerremains after the removing the first middle layer, and the methodfurther comprises: forming a second middle layer over and contacting thebottom layer; and forming a second top layer over the second middlelayer, wherein the second top layer comprises an additional photoresist. In an embodiment, the method includes performing light exposureson the first top layer and the second top layer using lithography maskshaving identical patterns. In an embodiment, the chemical solutionfurther comprises an organic solvent and water. In an embodiment, in theremoving the first middle layer, an etching selectivity of an etchingrate of the first middle layer to an etching rate of the bottom layer isgreater than about 100. In an embodiment, before the removing the firsttop layer, performing a light exposure and a development on the firsttop layer. In an embodiment, the chemical solution comprises both thequaternary ammonium hydroxide and the quaternary ammonium fluoride. Inan embodiment, the quaternary ammonium hydroxide comprises TMAH, and thequaternary ammonium fluoride comprises TMAF, and a ratio of a weightpercentage of the TMAH to a weight percentage of the TMAF is close toabout 1:3. In an embodiment, the chemical solution is further free fromNaOH, NH₄OH, and HF.

In accordance with some embodiments of the present disclosure, a methodincludes forming a first middle layer over a bottom layer, wherein thefirst middle layer comprises siloxane; and etching the first middlelayer, wherein the bottom layer remains as a blanket layer after thefirst middle layer is etched, and the etching the first middle layer isperformed in a wet etching process using a chemical solution comprising:a quaternary ammonium hydroxide; a quaternary ammonium fluoride; anorganic solvent; and water. In an embodiment, the method furtherincludes, after the first middle layer is etched, forming a secondmiddle layer comprising siloxane over the bottom layer; and patterningthe second middle layer. In an embodiment, after the etching the firstmiddle layer, substantially an entirety of the bottom layer remains. Inan embodiment, the organic solvent is configured to stabilize silanol.In an embodiment, the organic solvent comprises ethylene glycol.

In accordance with some embodiments of the present disclosure, achemical solution includes at least one of a quaternary ammoniumhydroxide and a quaternary ammonium fluoride; an organic solvent; andwater, wherein the chemical solution is free from KOH. In an embodiment,the chemical solution includes both the quaternary ammonium hydroxideand the quaternary ammonium fluoride. In an embodiment, the quaternaryammonium hydroxide comprises TMAH, and the quaternary ammonium fluoridecomprises TMAF. In an embodiment, a ratio of a weight percentage of theTMAH to a weight percentage of the TMAF is close to about 1:3. In anembodiment, the chemical solution is free from NaOH, NH₄OH, and HF. Inan embodiment, the organic solvent has a weight percentage in a rangebetween about 10 percent and about 15 percent.

In accordance with some embodiments of the present disclosure, a methodincludes forming a bottom layer; forming a first middle layer over thebottom layer; forming a first top layer over the first middle layer;performing a light exposure and a development on the first top layer;removing the first top layer; removing an entirety of the first middlelayer in a chemical solution, wherein the bottom layer remains after thefirst middle layer is removed; forming a second middle layer over thebottom layer; forming a second top layer; performing an additional lightexposure and an additional development on the second top layer; andetching the second middle layer and the bottom layer using the secondtop layer as an etching mask. In an embodiment, the chemical solutioncomprises both a quaternary ammonium hydroxide and a quaternary ammoniumfluoride. In an embodiment, the method includes transferring patterns inthe second middle layer and the bottom layer into an underlying low-kdielectric layer. In an embodiment, the method includes forming metallines in the underlying low-k dielectric layer, with the metal lineshaving patterns same as patterns formed in the second middle layer.

In accordance with some embodiments of the present disclosure, achemical solution includes an alkali; a fluoride; an organic solvent,wherein the organic solvent is solvable in water; and water having aweight percentage greater than about 70 percent in the chemicalsolution. In an embodiment, the chemical solution is free from KOH,NaOH, NH₄OH, and HF. In an embodiment, the alkali comprises a quaternaryammonium hydroxide, and the fluoride comprises a quaternary ammoniumfluoride. In an embodiment, the quaternary ammonium hydroxide comprisesTMAH, and the quaternary ammonium fluoride comprises TMAF.

In accordance with some embodiments of the present disclosure, achemical solution includes TMAH; TMAF, wherein a ratio of a weightpercentage of the TMAH to a weight percentage of the TMAF is close toabout 1:3; an organic solvent configured to stabilize silanol; andwater. In an embodiment, the organic solvent has a weight percentage ina range between about 10 percent and about 15 percent.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method comprising: forming a tri-layer, the tri-layer comprising: abottom layer; a first middle layer over the bottom layer; and a firsttop layer over the first middle layer, wherein the first top layercomprises a photo resist; removing the first top layer; and removing thefirst middle layer using a chemical solution, wherein the chemicalsolution is free from potassium hydroxide (KOH), and comprises at leastone of a quaternary ammonium hydroxide and a quaternary ammoniumfluoride.
 2. The method of claim 1, wherein the bottom layer remainsafter the removing the first middle layer, and the method furthercomprises: forming a second middle layer over and contacting the bottomlayer; and forming a second top layer over the second middle layer,wherein the second top layer comprises an additional photo resist. 3.The method of claim 2 further comprising performing light exposures onthe first top layer and the second top layer using lithography maskshaving identical patterns.
 4. The method of claim 1, wherein thechemical solution further comprises an organic solvent and water.
 5. Themethod of claim 1, wherein in the removing the first middle layer, anetching selectivity of an etching rate of the first middle layer to anetching rate of the bottom layer is greater than about
 100. 6. Themethod of claim 1 further comprising, before the removing the first toplayer, performing a light exposure and a development on the first toplayer.
 7. The method of claim 1, wherein the chemical solution comprisesboth the quaternary ammonium hydroxide and the quaternary ammoniumfluoride.
 8. The method of claim 7, wherein the quaternary ammoniumhydroxide comprises tetramethylammonium hydroxide (TMAH), and thequaternary ammonium fluoride comprises tetramethylammonium fluoride(TMAF), and a ratio of a weight percentage of the TMAH to a weightpercentage of the TMAF is close to about 1:3.
 9. The method of claim 1,wherein the chemical solution is further free from NaOH, NH₄OH, and HF.10. A method comprising: forming a first middle layer over a bottomlayer, wherein the first middle layer comprises siloxane; and etchingthe first middle layer, wherein the bottom layer remains as a blanketlayer after the first middle layer is etched, and the etching the firstmiddle layer is performed in a wet etching process using a chemicalsolution comprising: a quaternary ammonium hydroxide; a quaternaryammonium fluoride; an organic solvent; and water.
 11. The method ofclaim 10 further comprising: after the first middle layer is etched,forming a second middle layer comprising siloxane over the bottom layer;and patterning the second middle layer.
 12. The method of claim 11,wherein after the etching the first middle layer, substantially anentirety of the bottom layer remains.
 13. The method of claim 10,wherein the organic solvent is configured to stabilize silanol.
 14. Themethod of claim 10, wherein the organic solvent comprises ethyleneglycol. 15.-20. (canceled)
 21. A method comprising: forming a bottomlayer; forming a first middle layer over the bottom layer; forming afirst top layer over the first middle layer; performing a light exposureand a development on the first top layer; removing the first top layer;removing an entirety of the first middle layer in a chemical solution,wherein the bottom layer remains after the first middle layer isremoved; forming a second middle layer over the bottom layer; forming asecond top layer; performing an additional light exposure and anadditional development on the second top layer; and etching the secondmiddle layer and the bottom layer using the second top layer as anetching mask.
 22. The method of claim 21, wherein the chemical solutioncomprises both a quaternary ammonium hydroxide and a quaternary ammoniumfluoride.
 23. The method of claim 21 further comprising transferringpatterns in the second middle layer and the bottom layer into anunderlying low-k dielectric layer.
 24. The method of claim 23 furthercomprising forming metal lines in the underlying low-k dielectric layer,with the metal lines having patterns same as patterns formed in thesecond middle layer.
 25. The method of claim 21, wherein the chemicalsolution comprises ethylene glycol as an organic solvent.
 26. The methodof claim 21, wherein after the entirety of the first middle layer isremoved, substantially an entirety of the bottom layer remains.